The printing process commonly used in industries which require reproducing graphic material, the newspaper and book publishing industries, for example, deposits a uniform density of ink on paper whenever it is desired to print all or a portion of an image and deposits no ink when the absence of an image is desired.
The all or nothing process poses no problem when alphabetical or other alphanumeric characters are printed. However, when pictures such as photographs are printed, the problem of reproducing the continuous tones (i.e. light gradations) arises. This problem has been solved by transforming the continuous tones in the original image into a halftone image which comprises a large number of ink dots of various sizes. This is referred to as "screening" and is performed by projecting the image through a fine mesh screen onto a photographic medium. When the largest dots and the spaces on the paper between the dots are made small compared with the visual acuity of the human eye, the dots and the spaces between the dots fuse visually in the screened image, the eye believing it is seeing continuous tones.
However, in an automated system in which electronic image reproduction forms at least part of the process of converting a continuous original image into a halftone image, the necessity for switching from electronic to photographic techniques in order to produce halftone is a factor which adds to the cost and complexity of the process. An electronic photocomposition system which obviates this problem is disclosed in U.S. Pat. No. 3,465,199. The system disclosed therein translates the tonal information on an original transparency into a corresponding image on the face of a cathode ray tube. The halftone images are recorded on film and thereafter may be processed into a printing plate by well known techniques. Another system which eliminates the aforementioned photographic technique is disclosed in U.S. Pat. No. 3,646,262 which also discloses means to vary the size or shape of the halftone dots formed on a photosensitive member. The aforementioned systems are primarily concerned with reproducing, as halftone, a black and white original. Color reproduction requires the reproduction of many different colors and shades. The multitude of colors is produced in conventional printing processes by the three subtractive primary colors, cyan, magenta and yellow. For high-quality reproduction a fourth ink, black, is also utilized. For large-volume reproduction of an original color pattern, there is prepared a set of halftone printing plates, with each carrying a halftone image of one color component of the original pattern. The original pattern is reproduced by overprinting with each printing plate so that the three printing inks visually combine to produce the correct colors.
The printing plates needed for color printing may be derived by scanning the original pattern in an electronic color scanner machine as set forth in U.S. Pat. No. 3,622,690. The color scanner typically scans the original pattern with light and measures the tones or color in the pattern by filtering the scanned signal with red, blue, and green color filters. The amplitudes of the filtered signals indicate the color content of the original pattern. Since the color printing inks are not spectrally perfect and hence do not correspond exactly to the three subtractive colors, the filtered signals are corrected for these deficiencies by means of color correction circuits in the color scanner. The color corrected signals are utilized to modulate the light emitted from a laser to produce continuous tone color separations of the original pattern. The continuous tone color separations are then screened photographically and further processed to prepare the halftone printing plates. Alternately, screened color separations are directly provided without requiring a separate photographic screening step.
Other halftone techniques utilize variations of character generation schemes whereby various elements of a two-dimensional matrix are turned on or off to create various dot patterns and characteristics. Alternate techniques deflect a CRT beam or laser beam in such a mannner as to draw dots of various shapes and characteristics. The dots are then repeated spatially to generate a halftone grid.
Prior art systems may incorporate electronic schemes which generate a horizontal or vertical line halftone, the scheme utilizing a pulse width modulation technique. In particular, a reference signal, which may be triangular, sine, cosine, waveform, depending upon the desired amplitude to pulse width conversion characteristics, is applied to a voltage comparator which compares the reference signal with a signal representing the tonal values of a scanned original. The comparator output may be coupled, for example, to a cathode ray tube to control spot size. The aforementioned Pat. No. 3,465,199 is an example of such a system. U.S. Pat. No. 3,916,096 discloses a technique for constructing a two-dimension halftone by using an electronic line screening technique. In particular, a single reference signal is amplified in separate, parallel channels. The amplified outputs are compared with a video signal in separate comparators, the screened video output being switched between comparator outputs thereby providing two different dot line widths. The system described in this patent provides, in essence, a line halftone and not a continuously varying two-dimensional spot. Although the screened video output pattern may be recorded on a reproduction device, limited control of the shape of the dots generated and the angular relationship of the generated dots in relation to the scanning direction is provided.
The line halftone techniques set forth hereinabove for converting continuous tone originals into halftones do not provide the reproduction details required in many applications. Further, it would be desirable to adapt electronic halftone techniques to directly reproduce, or copy, a black and white or color original document either locally or at a remote location. Although black and white, and recently, color copiers, are commercially available, the techniques utilized therein provide reproductions which although satisfactory for most purposes, are limited in some respects. In particular, reproduction of continuous tone, black and white and color originals have not provided the details required in certain applications.
It would be desirable, therefore, if two-dimensional electronic halftone techniques can be provided for black and white and color copying processes which allow the shape and characteristics of the halftone dots to be easily controlled, provides for electronic screen simulation and angular rotation thereof to reduce Moire' pattern effects, is economical and reliable and which provides a reproduction or copy whose tonal characteristics are a substantial replica of that in the original.